ORIGIN OF THE MESODERM. 51 



unchanged, or even slightly increase in dimension, yet the growth of the embryo 

 causes that connection to appear relatively small. A connection of i or 2 mm. 

 equals at first the entire length of the embryo, but a connection of 4 or 5 mm. 

 seems small when the embryo is 100 or 200 mm. long. 



The relations of the embryo to the yolk in the anamniota are illustrated by 

 the accompanying figure 17, which represents a transverse section through a young 

 stage of the catfish (Amiurus). The section passes through the head of the 

 embryo and shows both eyes and the slender optic nerves, N.op, almost symmet- 

 rically cut on both sides. The yolk, Yk, is a a large mass heavily laden with yolk- 

 granules. Between the tissues of the embryo proper and of the yolk-sac there is 

 a direct continuity. Not only can the ectoderm, EC, be followed around from the 

 embryo over the yolk-sac, but also a layer of mesoderm. The part of the yolk-sac 

 which carries the yolk grains is, as above stated, a modification of the entoderm. 

 There is no amnion. 



Origin of the Mesoderm. 



The development of the primitive streak and groove is accompanied by the 

 appearance of the third or middle germ-layer, the mesoderm (Fig. 15, mes). As 

 shown in the section there figured, the three germ-layers are fused together under- 

 neath the primitive groove, and are there thicker than elsewhere. As we pass 

 laterally from the groove, the ectoderm and mesoderm both become thinner and are' 

 distinctly separated from one another. The entoderm consists of a single thin 

 layer of cells very closely connected with the mesoderm. The mesoderm occupies 

 at first only a small area in the immediate neighborhood of the primitive streak. 

 It grows rapidly, so that its edge extends farther and farther over the blastodermic 

 vesicle. The mesoderm is to be regarded as the product of the entoderm. Its 

 exact origin in mammals has not yet been adequately traced. We know, however, 

 that in birds, reptiles, and elasmobranchs the cells of the inner layer multiply 

 rapidly, so that the inner layer becomes more than one cell thick. The upper 

 cells soon split off from the lower and thus form themselves into the middle germ- 

 layer. The mesoderm therefore is said to be formed by delamination. It seems 

 probable that in mammals the process is the same. 



It may be mentioned that, according to Bonnet, the development of the meso- 

 derm in the sheep is not quite as above described. It can be first distinguished 

 at the stage when the primitive knot has appeared, and before the primitive streak 

 is developed. In the fresh specimen it is seen as a slight turbidity of the vesicular 

 walls just outside the edge of the shield (Fig. 18), while in the region of the shield 

 there is no middle layer whatever. By the time the primitive streak has appeared 

 in the sheep, the formation of the mesoderm has extended under the embryonic 

 shield, and the relations between the germ-layers then become essentially as above 

 described. 



The cells of the mesoderm are at first quite closely packed, but as the layer 



